The present invention relates to loop antennas which operate in close proximity to a human body. More specifically, the invention relates to loop antennas where the capacitive effect of the human body on the loop antenna is minimized and thus the impedance to current flow through the loop antenna is minimized.
Loop antennas are commonly used in telecommunications devices that are worn on or near the human body, such as pagers. Typically the loop antennas are configured to be "electrically small" such that the diameter of the loop antenna is much less than its operating wavelength. The loop antennas are advantageous in that they have a relatively low impedance which makes them somewhat immune to the capacitive effects of the human body. Nevertheless, much effort has been directed towards further minimizing these capacitive effects as illustrated in U.S. Pat. No. 5,072,231, and U.S. Pat. No. 4,327,444.
Electrically small loop antennas have an impedance which can be characterized by the equation: EQU Z=Rr+Rl+jWLl,
where Rr is the radiation resistance, Rl is the loss resistance and Ll is the loop inductance. Since the transmit power is proportional to the square of the antenna current times Rr, it is desirable to maximize Rr and minimize Rl and Ll in order to maximize the transmit power of the antenna. The capacitance of a human body affects the antenna performance by affecting the value of Rl and Ll. Thus, by minimizing the capacitive effect, Rl and Ll are minimized and the performance of the loop antenna is enhanced.
Furthermore, the low impedance associated with electrically small loop antennas makes it difficult to efficiently drive a current through the antenna, especially if the current is driven by a transistor having a low operating voltage. The problem is that the output impedance of the transistor is relatively high compared to the input impedance of the loop antenna thereby creating an impedance mismatch which makes it difficult to efficiently drive current. By decreasing the impedance mismatch between the transistor and the loop antenna, the antenna current is increased and the performance of the antenna is enhanced.
It is accordingly an object of the present invention to provide a novel antenna and method which minimizes the capacitive effect of a capacitive body.
A further object of the present invention is to provide a novel antenna where the current through the antenna is maximized by matching the antenna impedance with that of a transistor.
It is yet a further object of the present invention to provide a novel antenna and method where a loop antenna is oriented relative to a human body to maximize the antenna current.
It is still a further object of the present invention to provide a method of optimally operating a low impedance loop antenna.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.